BKL: revert back to the old spinlock implementation

The generic semaphore rewrite had a huge performance regression on AIM7
(and potentially other BKL-heavy benchmarks) because the generic
semaphores had been rewritten to be simple to understand and fair.  The
latter, in particular, turns a semaphore-based BKL implementation into a
mess of scheduling.

The attempt to fix the performance regression failed miserably (see the
previous commit 00b41ec2611dc98f87f30753ee00a53db648d662 'Revert
"semaphore: fix"'), and so for now the simple and sane approach is to
instead just go back to the old spinlock-based BKL implementation that
never had any issues like this.

This patch also has the advantage of being reported to fix the
regression completely according to Yanmin Zhang, unlike the semaphore
hack which still left a couple percentage point regression.

As a spinlock, the BKL obviously has the potential to be a latency
issue, but it's not really any different from any other spinlock in that
respect.  We do want to get rid of the BKL asap, but that has been the
plan for several years.

These days, the biggest users are in the tty layer (open/release in
particular) and Alan holds out some hope:

  "tty release is probably a few months away from getting cured - I'm
   afraid it will almost certainly be the very last user of the BKL in
   tty to get fixed as it depends on everything else being sanely locked."

so while we're not there yet, we do have a plan of action.

Tested-by: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Alexander Viro <viro@ftp.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 23 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 58fb8af15776..c51b6565e07c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4567,8 +4567,6 @@ EXPORT_SYMBOL(schedule);
 asmlinkage void __sched preempt_schedule(void)
 {
 	struct thread_info *ti = current_thread_info();
-	struct task_struct *task = current;
-	int saved_lock_depth;
 
 	/*
 	 * If there is a non-zero preempt_count or interrupts are disabled,
@@ -4579,16 +4577,7 @@ asmlinkage void __sched preempt_schedule(void)
 
 	do {
 		add_preempt_count(PREEMPT_ACTIVE);
-
-		/*
-		 * We keep the big kernel semaphore locked, but we
-		 * clear ->lock_depth so that schedule() doesnt
-		 * auto-release the semaphore:
-		 */
-		saved_lock_depth = task->lock_depth;
-		task->lock_depth = -1;
 		schedule();
-		task->lock_depth = saved_lock_depth;
 		sub_preempt_count(PREEMPT_ACTIVE);
 
 		/*
@@ -4609,26 +4598,15 @@ EXPORT_SYMBOL(preempt_schedule);
 asmlinkage void __sched preempt_schedule_irq(void)
 {
 	struct thread_info *ti = current_thread_info();
-	struct task_struct *task = current;
-	int saved_lock_depth;
 
 	/* Catch callers which need to be fixed */
 	BUG_ON(ti->preempt_count || !irqs_disabled());
 
 	do {
 		add_preempt_count(PREEMPT_ACTIVE);
-
-		/*
-		 * We keep the big kernel semaphore locked, but we
-		 * clear ->lock_depth so that schedule() doesnt
-		 * auto-release the semaphore:
-		 */
-		saved_lock_depth = task->lock_depth;
-		task->lock_depth = -1;
 		local_irq_enable();
 		schedule();
 		local_irq_disable();
-		task->lock_depth = saved_lock_depth;
 		sub_preempt_count(PREEMPT_ACTIVE);
 
 		/*
@@ -5853,8 +5831,11 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
 	spin_unlock_irqrestore(&rq->lock, flags);
 
 	/* Set the preempt count _outside_ the spinlocks! */
+#if defined(CONFIG_PREEMPT)
+	task_thread_info(idle)->preempt_count = (idle->lock_depth >= 0);
+#else
 	task_thread_info(idle)->preempt_count = 0;
-
+#endif
 	/*
 	 * The idle tasks have their own, simple scheduling class:
 	 */